In ordinary real time signals such as voice or picture, there are usually silent periods which typically occupy from 20% to 40% of the time. Such a signal can be converted into digital signal for digital trnsmission and reproduction. For such a system, the signal can be processed by means of pulse code modulation, difference pulse code modulation, delta modulation or adaptive predictive difference code modulation. In these modulation methods, an analog signal is sampled at different time intervals and quantized into many levels.
If the signal has a fixed pattern such as that in a music box or recordings, the signal can be stored in a memory and played back when instructed. For such an application, it is desirable to conserve the memory capacity for economical reasons.